TESS is a space telescope in NASA's
Explorer program, designed to search for extrasolar planets using the
transit method. The primary mission objective for TESS is to survey the
brightest stars near the Earth for transiting exoplanets over a
two-year period. The TESS project will use an array of wide-field
cameras to perform an all-sky survey. It will scan nearby stars for
exoplanets. 1)2)3)

In the first-ever spaceborne all-sky
transit survey, TESS will identify planets ranging from Earth-sized to
gas giants, orbiting a wide range of stellar types and orbital
distances. The principal goal of the TESS mission is to detect small
planets with bright host stars in the solar neighborhood, so that
detailed characterizations of the planets and their atmospheres can be
performed.

TESS will monitor the brightnesses
of more than 200,000 stars during a two year mission, searching for
temporary drops in brightness caused by planetary transits. Transits
occur when a planet's orbit carries it directly in front of its parent
star as viewed from Earth. TESS is expected to catalog more than 1,500
transiting exoplanet candidates, including a sample of ~500 Earth-sized
and ‘Super Earth’ planets, with radii less than twice that
of the Earth. TESS will detect small rock-and-ice planets orbiting a
diverse range of stellar types and covering a wide span of orbital
periods, including rocky worlds in the habitable zones of their host
stars.

The lead institution for TESS is MIT
(Massachusetts Institute of Technology), with George Ricker as PI
(Principal Investigator). The MIT/LL (Lincoln Laboratory) is
responsible for the cameras, including the lens assemblies, detector
assemblies, lens hoods, and camera mount. NASA/GSFC (Goddard Space
Flight Center) provides project management, systems engineering, and
safety and mission assurance. Orbital ATK (OA) builds and operates the
spacecraft. The mission is operated from the OA Mission Operations
Center.

The TESS Science Center, which
analyzes the science data and organizes the co-investigators,
collaborators, and working groups (with members from many institutions)
is a partnership among MIT's Physics Department and Kavli Institute for
Astrophysics and Space Research, the SAO (Smithsonian Astrophysical
Observatory), and the NASA Ames Research Center. The raw and processed
data are archived at the Mikulski Archive for Space Telescopes, at the
Space Telescope Science Institute.

Figure 1: This animation shows
how a dip in the observed brightness of a star may indicate the
presence of a planet passing in front of it, an occurrence known as a
transit (image credit: NASA/GSFC)

Some background: TESS is a
NASA-based mission, selected in 2013 as an astrophysics mission in the
Explorers Program. TESS has a long history, beginning as a small,
privately funded mission in 2006. It started with financial backing
from private companies, including Google, the Kavli Foundation, and
donors at MIT. This all changed in 2008, when MIT proposed TESS as an
official NASA astrophysics mission, re-structuring it as a SMEX (Small
Explorer) Class Mission. After not being selected in this competitive
process for NASA resources, TESS proposed again in 2010 as a NASA
Explorer (EX) Class Mission. TESS is the first of this new
classification of Explorer missions. In 2013, TESS was successful in
the proposal process and NASA began the development of the project.
MIT's Kavli Institute of Technology for Astrophysics (MKI) has remained
as an original partner in the current TESS mission, joining NASA in the
next search for new worlds. 4)

TESS will concentrate on stars less
than 300 light-years away and 30-100 times brighter than those surveyed
by the Kepler satellite; thus,TESS planets should be far easier to
characterize with follow-up observations. The brightness of these
target stars will allow researchers to use spectroscopy, the study of
the absorption and emission of light, to determine a planet’s
mass, density and atmospheric composition. Water, and other key
molecules, in its atmosphere can give us hints about a planets’
capacity to harbor life. These follow-up observations will provide
refined measurements of the planet masses, sizes, densities, and
atmospheric properties. 5)

TESS will provide prime targets for
further, more detailed characterization with the James Webb Space
Telescope (JWST), as well as other large ground-based and space-based
telescopes of the future. TESS's legacy will be a catalog of the
nearest and brightest stars hosting transiting exoplanets, which will
comprise the most favorable targets for detailed investigations in the
coming decades.

The Kepler project has provided
ground-breaking new insights into the population of exoplanets in our
galaxies; among the discoveries made using data from Kepler is the fact
that the most common members of the exoplanet family are Earths and
Super-Earths. However, the majority of exoplanets found by Kepler orbit
faraway, faint stars. This, combined with the relatively small size of
Earths and Super-Earths, means that there is currently a dearth of such
planets that can be characterized with follow-up observations.

“TESS is
opening a door for a whole new kind of study,” said Stephen
Rinehart, TESS project scientist at NASA/GSFC (Goddard Space Flight
Center) in Greenbelt, Maryland, which manages the mission.
“We’re going to be able study individual planets and start
talking about the differences between planets. The targets TESS finds
are going to be fantastic subjects for research for decades to come.
It’s the beginning of a new era of exoplanet research.”

Through the TESS Guest Investigator Program,
the worldwide scientific community will be able to participate in
investigations outside of TESS’s core mission, enhancing and
maximizing the science return from the mission in areas ranging from
exoplanet characterization to stellar astrophysics and solar system
science (Ref. 6).

“I don’t think we know
everything TESS is going to accomplish,” Rinehart said. “To
me, the most exciting part of any mission is the unexpected result, the
one that nobody saw coming.”

TESS is designed to:

• Focus on Earth and Super-Earth size planets

• Cover 400 X larger sky area than Kepler

• Span stellar spectral types of F5 to M5

Transiting exoplanets allow the project to observe the following for those planets that transit nearby bright stars:

Figure 2: Left: Sizes and
orbital periods of planets with host stars brighter than J = 10. Right:
Currently known planets, including those from the Kepler and CoRoT
missions as well as ground-based surveys. Figure on the right now
including the simulated population of TESS exoplanet detections (image
credit: NASA)

TESS will tile the sky with 26 observation sectors:

• At least 27 days staring at each 24° x 96° sector

• Brightest 100,000 stars at 1-minute cadence

• Full frame images with 30-minute cadence

• Map Northern hemisphere in first year

• Map Southern hemisphere in second year

• Sectors overlap at ecliptic
poles for sensitivity to smaller and longer period planets in JWST CVZ
(Continuous Viewing Zone).

Figure 3: Illustration of the
TESS (Transiting Exoplanet Survey Telescope) in front of a lava planet
orbiting its host star. TESS will identify thousands of potential new
planets for further study and observation (image credit: NASA/GSFC) 6)

Spacecraft:

The TESS mission is based on
Orbital's LEOStar-2 platform, a flexible, high-performance spacecraft
for space and Earth science, remote sensing and other applications.
LEOStar-2 can accommodate various instrument interfaces, deliver up to
2 kW orbit average payload power, and support payloads up to 500 kg.
Performance options include redundancy, propulsion capability, high
data rate communications, and high-agility/high-accuracy pointing. TESS
will be the eighth LEOStar-2 based spacecraft built for NASA. Previous
missions include SORCE, GALEX, AIM, NuSTAR and the OCO-2 spacecraft.

The LEOStar-2 bus has a three-axis
controlled, zero-momentum attitude control system, and two deployed
solar array wings. The total observatory power draw is estimated to be
290 W, and the solar arrays are capable of producing 415 W. To achieve
fine pointing, the spacecraft uses four reaction wheels and
high-precision quaternions produced by the science cameras. The
transmitter has a body-fixed high-gain antenna with a diameter of 0.7
m, a power of 2 W and a data rate of 100 Mbit/s. This is sufficient to
downlink the science data during 4 hr intervals at each perigee.

DHU (Data Handling Unit): The DHU is
a Space Micro Image Processing Computer (IPC-7000) which consists of
six boards: an IPC (Image Processing Computer), which contains two
Virtex-7 FPGAs (Field Programmable Gate Arrays) that serve as
interfaces to the four cameras and perform high-speed data processing;
a Proton 400 k single board computer, which is responsible for
commanding, communicating with the spacecraft master avionics unit, and
interfacing with the Ka-band transmitter; two 192 GB SSB (Solid-State
Buffer) cards for mass data storage; an analog I/O power switch board
to control instrument power; and a power supply board for the DHU.

The CCDs (Charge Coupled Devices)
produce a continuous stream of images with an exposure time of 2
seconds. These are received by the FPGAs on the IPC, and summed into
consecutive groups of 60, giving an effective exposure time of 2
minutes. During science operations, the DHU performs real-time
processing of data from the four cameras, converting CCD images into
the data products required for ground post-processing. A primary data
product is a collection of subarrays (nominally 10 x 10 pixels)
centered on preselected target stars. The Proton400 k extracts these
subarrays from each 2 min summed image, compresses them and stores them
in the SSB prior to encapsulation as CCSDS packets for the Ka-band
transmitter. Full frame images are also stacked every 30 minutes and
stored in the SSB. Data from the SSB are downlinked every 13.7 days at
perigee.

At perigee, science operations are
interrupted for no more than 16 hours to point TESS 's antenna toward
Earth, downlink data, and resume observing. This includes a nominal 4
hr period for Ka-band science data downlink using NASA's DSN (Deep
Space Network). In addition, momentum unloading is occasionally needed
due to the ~1.5 N m of angular momentum build-up induced by solar
radiation pressure. For this purpose TESS uses its hydrazine thrusters.

• February 15, 2018: NASA's
TESS satellite has arrived in Florida to begin preparations for launch.
TESS was delivered Feb. 12 aboard a truck from Orbital ATK in Dulles,
Virginia, where it spent 2017 being assembled and tested. Over the next
month, the spacecraft will be prepped for launch at Kennedy's Payload
Hazardous Servicing Facility (PHSF). 8)

Figure 6: TESS arrives at
NASA’s Kennedy Space Center, where it will undergo final
preparations for launch. Launch is scheduled for no earlier than April
16, pending range approval (image credit: NASA’s Kennedy Space
Center)

Launch: The TESS spacecraft
was launched on 18 April 2018 (22:51 UTC) from the Cape Canaveral Air
Force Station in Florida, SLC-40 (Space Launch Complex-40). The launch
provider was SpaceX using the Falcon-9 V1.1 launch vehicle.9)10)11)12)13)

Following stage separation, SpaceX
successfully landed Falcon 9’s first stage on the “Of
Course I Still Love You” droneship in the Atlantic Ocean. —
After TESS was released the satellite deployed its solar arrays, and it
will take 60 days for the satellite to attain its proper orbit.

Orbit: HEO (Highly Elliptical Orbit) with a nominal perigee of 17 RE (Earth radii) equivalent to 108,000 km, and a nominal apogee of 59 RE or 373,000 km, inclination = 28.5º, period of 13.7 days in 2:1 resonance with the Moon's orbit.

The orbit remains
above the Earth's radiation belts, leading to a relatively
low-radiation environment with a mission total ionizing dose of <1
krad. The nearly constant thermal environment ensures that the CCDs
will operate near -75ºC, with temperature variations
<0.1ºC /hr for 90% of the orbit, and <2ºC/hr throughout
the entire orbit (Ref. 3).

This orbit can be reached efficiently using a small supplemental propulsion system (ΔV ~3 km/s) augmented by a lunar gravity assist.
The specific path to the orbit will depend on the launch date and
launch vehicle. In a nominal scenario (illustrated in Figure 7),
TESS is launched from Cape Canaveral into a parking orbit with an
equatorial inclination of 28.5º. The apogee is raised to 400,000
km after two additional burns by the spacecraft hydrazine system, one
at perigee of the first phasing orbit, and another at perigee of the
second phasing orbit. An adjustment is made at third perigee, before a
lunar flyby raises the ecliptic inclination to about 40º. A final
period-adjust maneuver establishes the desired apogee and the 13.7 day
period. The final orbit is achieved about 60 days after launch, and
science operations begin soon afterward.

The orbital period and semimajor
axis are relatively constant, with long-term exchanges of eccentricity
and inclination over a period of order 8-12 years (driven by a
Kozai-like mechanism)14). There are also short-term oscillations with a period of six months caused by solar perturbations ( Figure 8). The orbit is stable on the time scale of decades, or more, and requires no propulsion for station-keeping. Table 2 lists a number of advantages of this type of orbit for TESS.

Table 2: Characteristics of the TESS spacecraft orbit and comparisons to a low-Earth orbit

Figure 8:
Calculated time variations in the elements of the nominal TESS mission
orbit. The units of each curve are specified in the legend; AOP
(Argument of Perigee), GEO (Geosynchronous Earth Orbit), image credit:
TESS Team

Mission status:

• May 13, 2020: Astronomers
have detected elusive pulsation patterns in dozens of young, rapidly
rotating stars thanks to data from NASA’s Transiting Exoplanet
Survey Satellite (TESS). The discovery will revolutionize
scientists’ ability to study details like the ages, sizes and
compositions of these stars — all members of a class named for
the prototype, the bright star Delta Scuti. 15)

- “Delta Scuti stars clearly
pulsate in interesting ways, but the patterns of those pulsations have
so far defied understanding,” said Tim Bedding, a professor of
astronomy at the University of Sydney. “To use a musical analogy,
many stars pulsate along simple chords, but Delta Scuti stars are
complex, with notes that seem to be jumbled. TESS has shown us
that’s not true for all of them.”

- A paper describing the findings, led by Bedding, appears in the May 14 issue of the journal Nature and is now available online. 16)

Figure 9: Watch the pulsations
of a Delta Scuti star! In this illustration, the star changes in
brightness when internal sound waves at different frequencies cause
parts of the star to expand and contract. In one pattern, the whole
star expands and contracts, while in a second, opposite hemispheres
swell and shrink out of sync. In reality, a single star exhibits many
pulsation patterns that can tell astronomers about its age, composition
and internal structure. The exact light variations astronomers observe
also depend on how the star's spin axis angles toward us. Delta Scuti
stars spin so rapidly they flatten into ovals, which jumbles these
signals and makes them harder to decode. Now, thanks to NASA's
Transiting Exoplanet Survey Satellite, astronomers are deciphering some
of them (video credit: NASA's Goddard Space Flight Center)

- Geologists studying seismic waves
from earthquakes figured out Earth’s internal structure from the
way the reverberations changed speed and direction as they traveled
through it. Astronomers apply the same principle to study the interiors
of stars through their pulsations, a field called asteroseismology.

- Sound waves travel through a
star’s interior at speeds that change with depth, and they all
combine into pulsation patterns at the star’s surface.
Astronomers can detect these patterns as tiny fluctuations in
brightness and use them to determine the star’s age, temperature,
composition, internal structure and other properties.

- Delta Scuti stars are between 1.5
and 2.5 times the Sun’s mass. They’re named after Delta
Scuti, a star visible to the human eye in the southern constellation Scutum
that was first identified as variable in 1900. Since then, astronomers
have identified thousands more like Delta Scuti, many with NASA’s Kepler space telescope, another planet-hunting mission that operated from 2009 to 2018.

- But scientists have had trouble
interpreting Delta Scuti pulsations. These stars generally rotate once
or twice a day, at least a dozen times faster than the Sun. The rapid
rotation flattens the stars at their poles and jumbles the pulsation
patterns, making them more complicated and difficult to decipher.

- To determine if order exists in
Delta Scuti stars’ apparently chaotic pulsations, astronomers
needed to observe a large set of stars multiple times with rapid
sampling. TESS monitors large swaths of the sky for 27 days at a time,
taking one full image every 30 minutes with each of its four cameras.
This observing strategy allows TESS to track changes in stellar
brightness caused by planets passing in front of their stars, which is
its primary mission, but half-hour exposures are too long to catch the
patterns of the more rapidly pulsating Delta Scuti stars. Those changes
can happen in minutes.

Figure 10: Sound waves bouncing
around inside a star cause it to expand and contract, which results in
detectable brightness changes. This animation depicts one type of Delta
Scuti pulsation — called a radial mode — that is driven by
waves (blue arrows) traveling between the star’s core and
surface. In reality, a star may pulsate in many different modes,
creating complicated patterns that enable scientists to learn about its
interior (image credit: NASA's Goddard Space Flight Center)

- But TESS also captures snapshots
of a few thousand pre-selected stars — including some Delta Scuti
stars — every two minutes. When Bedding and his colleagues began
sorting through the measurements, they found a subset of Delta Scuti
stars with regular pulsation patterns. Once they knew what to look for,
they searched for other examples in data from Kepler, which used a
similar observing strategy. They also conducted follow-up observations
with ground-based telescopes, including one at the W.M. Keck Observatory in Hawaii and two in the global Las Cumbres Observatory network. In total, they identified a batch of 60 Delta Scuti stars with clear patterns.

- “This really is a
breakthrough. Now we have a regular series of pulsations for these
stars that we can understand and compare with models,” said
co-author Simon Murphy, a postdoctoral researcher at the University of
Sydney. “It’s going to allow us to measure these stars
using asteroseismology in a way that we’ve never been able to do.
But it’s also shown us that this is just a stepping-stone in our
understanding of Delta Scuti stars.”

- Pulsations in the well-behaved
Delta Scuti group fall into two major categories, both caused by energy
being stored and released in the star. Some occur as the whole star
expands and contracts symmetrically. Others occur as opposite
hemispheres alternatively expand and contract. Bedding’s team
inferred the alterations by studying each star’s fluctuations in
brightness.

- The data have already helped settle a debate over the age of one star, called HD 31901,
a member of a recently discovered stream of stars orbiting within our
galaxy. Scientists placed the age of the overall stream at 1 billion years,
based on the age of a red giant they suspected belonged to the same
group. A later estimate, based on the rotation periods of other members
of the stellar stream, suggested an age of only about 120 million years. Bedding’s team used the TESS observations to create an asteroseismic model of HD 31901 that supports the younger age.

Figure 11: Hear the rapid beat
of HD 31901, a Delta Scuti star in the southern constellation Lepus.
The sound is the result of 55 pulsation patterns TESS observed over 27
days sped up by 54,000 times. Delta Scuti stars have long been known
for their apparently random pulsations, but TESS data show that some,
like HD 31901, have more orderly patterns (video credits: NASA's
Goddard Space Flight Center and Simon Murphy, University of Sydney)

- "Delta Scuti stars have been
frustrating targets because of their complicated oscillations, so this
is a very exciting discovery," said Sarbani Basu, a professor of
astronomy at Yale University in New Haven, Connecticut, who studies
asteroseismology but was not involved in the study. "Being able to find
simple patterns and identify the modes of oscillation is game changing.
Since this subset of stars allows normal seismic analyses, we will
finally be able to characterize them properly."

- The team thinks their set of 60
stars has clear patterns because they’re younger than other Delta
Scuti stars, having only recently settled into producing all of their
energy through nuclear fusion in their cores. The pulsations occur more
rapidly in the fledgling stars. As the stars age, the frequency of the
pulsations slows, and they become jumbled with other signals.

- Another factor may be
TESS’s viewing angle. Theoretical calculations predict that a
spinning star’s pulsation patterns should be simpler when its
rotational pole faces us instead of its equator. The team’s TESS
data set included around 1,000 Delta Scuti stars, which means that some
of them, by chance, must be viewed close to pole-on.

- Scientists will continue to develop their models as TESS begins taking full images every 10 minutes
instead of every half hour in July. Bedding said the new observing
strategy will help capture the pulsations of even more Delta Scuti
stars.

- “We knew when we designed
TESS that, in addition to finding many exciting new exoplanets, the
satellite would also advance the field of asteroseismology,” said
TESS Principal Investigator George Ricker at the Massachusetts
Institute of Technology’s Kavli Institute for Astrophysics and Space Research in Cambridge. “The mission has already found a new type of star that pulsates on one side only and has unearthed new facts about well-known stars.
As we complete the initial two-year mission and commence the extended
mission, we’re looking forward to a wealth of new stellar
discoveries TESS will make.”

- TESS is a NASA Astrophysics
Explorer mission led and operated by MIT in Cambridge, Massachusetts,
and managed by NASA's Goddard Space Flight Center. Additional partners
include Northrop Grumman, based in Falls Church, Virginia; NASA’s
Ames Research Center in California’s Silicon Valley; the
Harvard-Smithsonian Center for Astrophysics in Cambridge,
Massachusetts; MIT’s Lincoln Laboratory; and the Space Telescope
Science Institute in Baltimore. More than a dozen universities,
research institutes and observatories worldwide are participants in the
mission.

• March 9, 2020: Scientists at
the Center for Astrophysics | Harvard & Smithsonian have, for the
first time, measured the orbital tilt of an exoplanet younger than 45
million years. While observing DS Tuc Ab—a recently discovered,
young, Neptune-sized planet with an orbital period of eight
days—scientists developed new modeling techniques to take stellar
obliquity measurements and demographic information about the planet. 17)

- "The discovery of DS Tuc Ab in
2019 gave us a unique opportunity to take measurements of a planet
around a very young star very soon after the planet's formation," said
George Zhou, astronomer at CfA. "This planet is only 40 million years
old; by comparison, our Solar System is 5 billion years old. We've
never had a planet so young that we can study in this fashion before."

Figure 12: At 40 million years
old, DS Tuc Ab is now the youngest planet for which scientists have
measured orbital tilt. Scientists used the young star as a proving
ground for new modeling techniques measuring stellar obliquity and
planetary demographics (image credit: M. Weiss)

- After a few billion years have
passed planets change, making it more difficult for scientists to
answer questions about the formation, life and maturation of planets.
"A lot of things can happen between when a planet is formed and when we
see them. The vast majority of planets we find are already mature and
we don't know what they were like when they were young," said Zhou.
"We've already learned that unlike other planets, DS Tuc Ab didn't
pinball, or get flung into, its star system. That opens up many other
possibilities for other similar, young exoplanets, and may help us to
better understand older planets we already know about."

- According to Zhou, the host star,
DS Tuc A, was covered up to 40% in star spots, making observation and
analysis of the young planet difficult. "Young stars don’t behave
nicely, and this is a really young star," said Zhou. "It is very
active, and the star spots initially made it difficult to take accurate
data since the planet was crossing our line of sight across the face of
the star."

- To combat these challenges and
characterize the planet and star system, scientists developed a new
technique for simultaneously modeling the many different factors
involved, allowing them to better track the young planet in its orbit.

- "We had to infer how many spots
there were, their size, and their color. Each time we'd add a star
spot, we'd check its consistency with everything we already knew about
the planet," said David Latham, CfA. "As TESS finds more young stars
like DS Tuc A, where the shadow of a transiting planet is hidden by
variations due to star spots, this new technique for uncovering the
signal of the planet will lead to a better understanding of the early
history of planets in their infancy."

- DS Tuc Ab was first discovered by
scientists at Dartmouth, CfA, and MIT, using data from NASA's TESS
mission in 2019. "We were excited when we first saw this planet's
signal," said Dr. Elisabeth Newton, Assistant Professor, Dartmouth.
"The star is bright and young, and we knew it would offer exciting
possibilities for in-depth investigations like this one." A parallel
discovery paper was published by scientists at INAF—the National
Institute for Astrophysics in Italy—the same year.

- Zhou and
scientists from the CfA began observing the planet in August 2019,
using the Planet Finder Spectrograph on the Magellan Telescope in
Chile. Results from the study will be published in the Astrophysical
Journal Letters. A companion study from Benjamin Montet (U. New South
Wales) et al. will be published in the Astronomical Journal. 18)

• January 7, 2020: Astronomers
using data from NASA’s Transiting Exoplanet Survey Satellite
(TESS) have shown that Alpha Draconis, a well-studied star visible to
the naked eye, and its fainter companion star regularly eclipse each
other. While astronomers previously knew this was a binary system, the
mutual eclipses came as a complete surprise. 19)

- “The first question that
comes to mind is ‘how did we miss this?’” said Angela
Kochoska, a postdoctoral researcher at Villanova University in
Pennsylvania who presented the findings at the 235th meeting of the
American Astronomical Society in Honolulu on Jan. 6. “The
eclipses are brief, lasting only six hours, so ground-based
observations can easily miss them. And because the star is so bright,
it would have quickly saturated detectors on NASA’s Kepler
observatory, which would also mask the eclipses.”

Figure 13: This animation
illustrates a preliminary model of the Thuban system, now known to be
an eclipsing binary thanks to data from NASA’s Transiting
Exoplanet Survey Satellite (TESS). The stars orbit every 51.4 days at
an average distance slightly greater than Mercury’s distance from
the Sun. We view the system about three degrees above the stars’
orbital plane, so they undergo mutual eclipses, but neither is ever
completely covered up by its partner. The primary star is 4.3 times
bigger than the Sun and has a surface temperature around 17,500º
Fahrenheit (9,700ºC), making it 70% hotter than our Sun. Its
companion, which is five times fainter, is most likely half the
primary’s size and 40% hotter than the Sun. Thuban, also called
Alpha Draconis, is located about 270 light-years away in the northern
constellation Draco [video credit: NASA’s Goddard Space Flight
Center/Chris Smith (USRA)]

- The system ranks among the
brightest-known eclipsing binaries where the two stars are widely
separated, or detached, and only interact gravitationally. Such systems
are important because astronomers can measure the masses and sizes of
both stars with unrivaled accuracy.

- Alpha Draconis, also known as Thuban, lies about 270 light-years away in the northern constellation Draco.
Despite its “alpha” designation, it shines as Draco’s
fourth-brightest star. Thuban’s fame arises from a historical
role it played some 4,700 years ago, back when the earliest pyramids
were being built in Egypt.

- At that time, it appeared as the
North Star, the one closest to the northern pole of Earth’s spin
axis, the point around which all of the other stars appear to turn in
their nightly motion. Today, this role is played by Polaris, a brighter
star in the constellation Ursa Minor. The change happened because
Earth’s spin axis performs a cyclic 26,000-year wobble, called
precession, that slowly alters the sky position of the rotational pole.

- TESS monitors large swaths of the
sky, called sectors, for 27 days at a time. This long stare allows the
satellite to track changes in stellar brightness. While NASA’s
newest planet hunter mainly seeks dimmings caused by planets crossing
in front of their stars, TESS data can be used to study many other
phenomena as well.

- A 2004 report suggested that
Thuban displayed small brightness changes that cycled over about an
hour, suggesting the possibility that the system’s brightest star
was pulsating.

- To check this, Timothy Bedding,
Daniel Hey, and Simon Murphy at the University of Sydney, Australia,
and Aarhus University, Denmark, turned to TESS measurements. In
October, they published a paper that described the discovery of eclipses by both stars and ruling out the existence of pulsations over periods less than eight hours.

- Now Kochoska is working with Hey
to understand the system in greater detail. “I've been
collaborating with Daniel to model the eclipses and advising on how to
bring together more data to better constrain our model.” Kochoska
explained. “The two of us took different approaches to modeling
the system, and we hope our efforts will result in its full
characterization.”

- As known from earlier studies,
the stars orbit every 51.4 days at an average distance of about 61
million km, slightly more than Mercury’s distance from the Sun.
The current preliminary model shows that we view the system about three
degrees above the stars’ orbital plane, which means neither star
completely covers the other during the eclipses. The primary star is
4.3 times bigger than the Sun and has a surface temperature around
17,500 ºF (9,700ºC), making it 70% hotter than our Sun. Its
companion, which is five times fainter, is most likely half the
primary’s size and 40% hotter than the Sun.

- “Discovering eclipses in a
well-known, bright, historically important star highlights how TESS
impacts the broader astronomical community,” said Padi Boyd, the
TESS project scientist at NASA’s Goddard Space Flight Center in
Greenbelt, Maryland. “In this case, the high precision,
uninterrupted TESS data can be used to help constrain fundamental
stellar parameters at a level we’ve never before achieved.”

Figure 14: The star Alpha
Draconis (circled), also known as Thuban, has long been known to be a
binary system. Now data from NASA's TESS show its two stars undergo
mutual eclipses (image credit: NASA/MIT/TESS)

• January 7, 2020: NASA's TESS
satellite has discovered its first Earth-size planet in its star's
habitable zone, the range of distances where conditions may be just
right to allow the presence of liquid water on the surface. Scientists
confirmed the find, called TOI 700 d, using NASA's Spitzer Space Telescope and have modeled the planet's potential environments to help inform future observations. 20)

- “TESS was designed and
launched specifically to find Earth-sized planets orbiting nearby
stars,” said Paul Hertz, astrophysics division director at NASA
Headquarters in Washington. “Planets around nearby stars are
easiest to follow-up with larger telescopes in space and on Earth.
Discovering TOI 700 d is a key science finding for TESS. Confirming the
planet’s size and habitable zone status with Spitzer is another
win for Spitzer as it approaches the end of science operations this
January."

Figure 15: NASA's Transiting
Exoplanet Survey Satellite (TESS) has discovered its first Earth-size
planet in its star's habitable zone, the range of distances where
conditions may be just right to allow the presence of liquid water on
the surface. Scientists confirmed the find, called TOI 700 d, using
NASA's Spitzer Space Telescope and have modeled the planet's potential
environments to help inform future observations (video credit: NASA's
Goddard Space Flight Center)

- TESS monitors large swaths of the
sky, called sectors, for 27 days at a time. This long stare allows the
satellite to track changes in stellar brightness caused by an orbiting
planet crossing in front of its star from our perspective, an event
called a transit.

- TOI 700 is a small, cool M dwarf star located just over 100 light-years away in the southern constellation Dorado.
It’s roughly 40% of the Sun’s mass and size and about half
its surface temperature. The star appears in 11 of the 13 sectors TESS
observed during the mission’s first year, and scientists caught
multiple transits by its three planets.

- The star was originally
misclassified in the TESS database as being more similar to our Sun,
which meant the planets appeared larger and hotter than they really
are. Several researchers, including Alton Spencer, a high school
student working with members of the TESS team, identified the error.

- “When we corrected the
star’s parameters, the sizes of its planets dropped, and we
realized the outermost one was about the size of Earth and in the
habitable zone,” said Emily Gilbert, a graduate student at the University of Chicago.
“Additionally, in 11 months of data we saw no flares from the
star, which improves the chances TOI 700 d is habitable and makes it
easier to model its atmospheric and surface conditions.”

- The innermost
planet, called TOI 700 b, is almost exactly Earth-size, is probably
rocky and completes an orbit every 10 days. The middle planet, TOI 700
c, is 2.6 times larger than Earth — between the sizes of Earth
and Neptune — orbits every 16 days and is likely a gas-dominated
world. TOI 700 d, the outermost known planet in the system and the only
one in the habitable zone, measures 20% larger than Earth, orbits every
37 days and receives from its star 86% of the energy that the Sun
provides to Earth. All of the planets are thought to be tidally locked
to their star, which means they rotate once per orbit so that one side
is constantly bathed in daylight.

- “Given the impact of this
discovery — that it is TESS’s first habitable-zone
Earth-size planet — we really wanted our understanding of this
system to be as concrete as possible,” Rodriguez said.
“Spitzer saw TOI 700 d transit exactly when we expected it to.
It’s a great addition to the legacy of a mission that helped
confirm two of the TRAPPIST-1 planets and identify five more.”

- The Spitzer data increased
scientists’ confidence that TOI 700 d is a real planet and
sharpened their measurements of its orbital period by 56% and its size
by 38%. It also ruled out other possible astrophysical causes of the
transit signal, such as the presence of a smaller, dimmer companion
star in the system.

- Rodriguez and his colleagues also used follow-up observations from a 1-meter ground-based telescope in the global Las Cumbres Observatory network to improve scientists’ confidence in the orbital period and size of TOI 700 c by 30% and 36%, respectively.

- Because TOI 700 is bright,
nearby, and shows no sign of stellar flares, the system is a prime
candidate for precise mass measurements by current ground-based
observatories. These measurements could confirm scientists’
estimates that the inner and outer planets are rocky and the middle
planet is made of gas.

- Future missions may be able to identify whether the planets have atmospheres and, if so, even determine their compositions.

- While the exact conditions on TOI
700 d are unknown, scientists can use current information, like the
planet’s size and the type of star it orbits, to generate
computer models and make predictions. Researchers at NASA’s
Goddard Space Flight Center in Greenbelt, Maryland, modeled 20
potential environments of TOI 700 d to gauge if any version would
result in surface temperatures and pressures suitable for habitability.

Figure 16: The three planets of
the TOI 700 system orbit a small, cool M dwarf star. TOI 700 d is the
first Earth-size habitable-zone world discovered by TESS (image credit:
NASA's Goddard Space Flight Center)

- Their 3D climate models examined
a variety of surface types and atmospheric compositions typically
associated with what scientists regard to be potentially habitable
worlds. Because TOI 700 d is tidally locked to its star, the
planet’s cloud formations and wind patterns may be strikingly
different from Earth’s.

- One simulation included an
ocean-covered TOI 700 d with a dense, carbon-dioxide-dominated
atmosphere similar to what scientists suspect surrounded Mars when it
was young. The model atmosphere contains a deep layer of clouds on the
star-facing side. Another model depicts TOI 700 d as a cloudless,
all-land version of modern Earth, where winds flow away from the night
side of the planet and converge on the point directly facing the star.

- When starlight
passes through a planet’s atmosphere, it interacts with molecules
like carbon dioxide and nitrogen to produce distinct signals, called
spectral lines. The modeling team, led by Gabrielle Engelmann-Suissa, a
Universities Space Research Association visiting research assistant at Goddard, produced simulated spectra for the 20 modeled versions of TOI 700 d.

- “Someday, when we have real
spectra from TOI 700 d, we can backtrack, match them to the closest
simulated spectrum, and then match that to a model,”
Engelmann-Suissa said. “It’s exciting because no matter
what we find out about the planet, it’s going to look completely
different from what we have here on Earth.”

- TESS is a NASA Astrophysics
Explorer mission led and operated by MIT in Cambridge, Massachusetts,
and managed by NASA's Goddard Space Flight Center. Additional partners
include Northrop Grumman, based in Falls Church, Virginia; NASA’s
Ames Research Center in California’s Silicon Valley; the
Harvard-Smithsonian Center for Astrophysics in Cambridge,
Massachusetts; MIT’s Lincoln Laboratory; and the Space Telescope
Science Institute in Baltimore. More than a dozen universities,
research institutes and observatories worldwide are participants in the
mission.

- The Jet Propulsion Laboratory in
Pasadena, California, manages the Spitzer Space Telescope mission for
NASA's Science Mission Directorate in Washington. Science operations
are conducted at the Spitzer Science Center at Caltech in Pasadena.
Space operations are based at Lockheed Martin Space in Littleton,
Colorado. Data are archived at the Infrared Science Archive housed at
IPAC at Caltech. Caltech manages JPL for NASA.

- The modeling work was funded
through the Sellers Exoplanet Environments Collaboration at Goddard, a
multidisciplinary collaboration that brings together experts to build
comprehensive and sophisticated computer models to better analyze
current and future exoplanet observations.

• December 4, 2019: Using data
from the TESS satellite, astronomers at the University of Maryland
(UMD), in College Park, Maryland, have captured a clear start-to-finish
image sequence of an explosive emission of dust, ice and gases during
the close approach of comet 46P/Wirtanen in late 2018. This is the most
complete and detailed observation to date of the formation and
dissipation of a naturally-occurring comet outburst. The team members
reported their results in the November 22 issue of The Astrophysical
Journal Letters. 21)22)23)

- “TESS spends nearly a month
at a time imaging one portion of the sky. With no day or night breaks
and no atmospheric interference, we have a very uniform, long-duration
set of observations,” said Tony Farnham, a research scientist in
the UMD Department of Astronomy and the lead author of the research
paper. “As comets orbit the Sun, they can pass through
TESS’ field of view. Wirtanen was a high priority for us because
of its close approach in late 2018, so we decided to use its appearance
in the TESS images as a test case to see what we could get out of it.
We did so and were very surprised!”

Figure 17: This animation shows
an explosive outburst of dust, ice and gases from comet 46P/Wirtanen
that occurred on September 26, 2018 and dissipated over the next 20
days. The images, from NASA’s TESS spacecraft, were taken every
three hours during the first three days of the outburst (image credit:
Farnham et al./NASA)

- “While TESS is a powerhouse
for discovering planets orbiting nearby, bright stars, its observing
strategy enables so much exciting additional science,” said TESS
project scientist Padi Boyd of NASA’s Goddard Space Flight Center
in Greenbelt, Maryland. “Since the TESS data are rapidly made
public through NASA’s Mikulski Archive for Space Telescopes
(MAST), it’s exciting to see scientists identifying which data
are of interest to them, and then doing all kinds of additional
serendipitous science beyond exoplanets.”

- Normal comet activity is driven
by sunlight vaporizing the ices near the surface of the nucleus, and
the outflowing gases drag dust off the nucleus to form the coma.
However, many comets are known to experience occasional spontaneous
outbursts that can significantly, but temporarily increase the comet's
activity. It is not currently known what causes outbursts, but they are
related to the conditions on the comet's surface. A number of potential
trigger mechanisms have been proposed, including a thermal event, in
which a heat wave penetrates into a pocket of highly volatile ices,
causing the ice to rapidly vaporize and produce an explosion of
activity, and a mechanical event, where a cliff collapses, exposing
fresh ice to direct sunlight. Thus, studies of the outburst behavior,
especially in the early brightening stages that are difficult to
capture, can help us understand the physical and thermal properties of
the comet.

- Although Wirtanen came closest to
Earth on December 16, 2018, the outburst occurred earlier in its
approach, beginning on September 26, 2018. The initial brightening of
the outburst occurred in two distinct phases, with an hour-long flash
followed by a more gradual second stage that continued to grow brighter
for another 8 hours. This second stage was likely caused by the gradual
spreading of comet dust from the outburst, which causes the dust cloud
to reflect more sunlight overall. After reaching peak brightness, the
comet faded gradually over a period of more than two weeks. Because
TESS takes detailed, composite images every 30 minutes, the team was
able to view each phase in exquisite detail.

- “With 20
days’ worth of very frequent images, we were able to assess
changes in brightness very easily. That’s what TESS was designed
for, to perform its primary job as an exoplanet surveyor,”
Farnham said. “We can’t predict when comet outbursts will
happen. But even if we somehow had the opportunity to schedule these
observations, we couldn’t have done any better in terms of
timing. The outburst happened mere days after the observations
started.”

- The team has generated a rough
estimate of how much material may have been ejected in the outburst,
about one million kilograms, which could have left a crater on the
comet of around 20 meters (about 65 feet) across. Further analysis of
the estimated particle sizes in the dust tail may help improve this
estimate. Observing more comets will also help to determine whether
multi-stage brightening is rare or commonplace in comet outbursts.

- TESS has also detected for the
first time Wirtanen’s dust trail. Unlike a comet’s
tail—the spray of gas and fine dust that follows behind a comet,
growing as it approaches the sun—a comet’s trail is a field
of larger debris that traces the comet’s orbital path as it
travels around the sun. Unlike a tail, which changes direction as it is
blown by the solar wind, the orientation of the trail stays more or
less constant over time.

- “The trail more closely
follows the orbit of the comet, while the tail is offset from it, as it
gets pushed around by the sun’s radiation pressure. What’s
significant about the trail is that it contains the largest
material,” said Michael Kelley, an associate research scientist
in the UMD Department of Astronomy and a co-author of the research
paper. “Tail dust is very fine, a lot like smoke. But trail dust
is much larger—more like sand and pebbles. We think comets lose
most of their mass through their dust trails. When the Earth runs into
a comet’s dust trail, we get meteor showers.”

- While the current study describes
initial results, Farnham, Kelley and their colleagues look forward to
further analyses of Wirtanen, as well as other comets in TESS’
field of view. “We also don’t know what causes natural
outbursts and that’s ultimately what we want to find,”
Farnham said. “There are at least four other comets in the same
area of the sky where TESS made these observations, with a total of
about 50 comets expected in the first two years’ worth of TESS
data. There’s a lot that can come of these data.”

- TESS is a NASA Astrophysics
Explorer mission led and operated by MIT in Cambridge, Massachusetts,
and managed by NASA's Goddard Space Flight Center. Additional partners
include Northrop Grumman, based in Falls Church, Virginia; NASA’s
Ames Research Center in California’s Silicon Valley; the
Harvard-Smithsonian Center for Astrophysics in Cambridge,
Massachusetts; MIT’s Lincoln Laboratory; and the Space Telescope
Science Institute in Baltimore. More than a dozen universities,
research institutes and observatories worldwide are participants in the
mission.

• November 5, 2019: The glow of
the Milky Way — our galaxy seen edgewise — arcs across a
sea of stars in a new mosaic of the southern sky produced from a year
of observations by NASA’s Transiting Exoplanet Survey Satellite (TESS). Constructed from 208 TESS images taken during the mission’s first year of science operations, completed on July 18, the southern panorama reveals both the beauty of the cosmic landscape and the reach of TESS’s cameras. 24)

- “Analysis of TESS data
focuses on individual stars and planets one at a time, but I wanted to
step back and highlight everything at once, really emphasizing the
spectacular view TESS gives us of the entire sky,” said Ethan
Kruse, a NASA Postdoctoral Program Fellow who assembled the mosaic at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.

Figure 18: NASA’s
Transiting Exoplanet Survey Satellite (TESS) spent a year imaging the
southern sky in its search for worlds beyond our solar system. Dive
into a mosaic of these images to see what TESS has found so far (video
credit: NASA’s Godard Space Flight Center)

- Within this scene, TESS has discovered 29 exoplanets, or worlds beyond our solar system, and more than 1,000 candidate planets astronomers are now investigating.

- TESS divided the southern sky
into 13 sectors and imaged each one of them for nearly a month using
four cameras, which carry a total of 16 charge-coupled devices (CCDs).
Remarkably, the TESS cameras capture a full sector of the sky every 30
minutes as part of its search for exoplanet transits. Transits occur
when a planet passes in front of its host star from our perspective,
briefly and regularly dimming its light. During the satellite’s
first year of operations, each of its CCDs captured 15,347 30-minute
science images. These images are just a part of more than 20 terabytes
of southern sky data TESS has returned, comparable to streaming nearly
6,000 high-definition movies.

Figure 19: The
plane of our Milky Way galaxy arcs across a starry landscape in this
detail of the TESS southern sky mosaic [image credit: NASA/MIT/TESS and
Ethan Kruse (USRA)]

• October 30, 2019: Using
asteroseismic data from NASA's TESS (Transiting Exoplanet Survey
Satellite) mission, an international team, led by Instituto de
Astrofisica e Ciencias do Espaco (IA) researcher Tiago Campante (Porto,
Portugal), studied the red-giant stars HD 212771 and HD 203949. These
are the first detections of oscillations in previously known
exoplanet-host stars by TESS. The result was published today in an
article in The Astrophysical Journal. 25)26)

- Tiago Campante (IA and Faculdade
de Ciencias da Universidade do Porto - FCUP) explains that detecting
these oscillations was only possible because: "TESS observations are
precise enough to allow measuring the gentle pulsations at the surfaces
of stars. These two fairly evolved stars also host planets, providing
the ideal testbed for studies of the evolution of planetary systems."

- Having determined the physical
properties of both stars, such as their mass, size and age, through
asteroseismology, the authors then focused their attention on the
evolutionary state of HD 203949. Their aim was to understand how its
planet could have avoided engulfment, since the envelope of the star
would have expanded well beyond the current planetary orbit during the
red-giant phase of evolution.

- Co-author Vardan Adibekyan (IA
and Universidade do Porto) comments: "This study is a perfect
demonstration of how stellar and exoplanetary astrophysics are linked
together. Stellar analysis seems to suggest that the star is too
evolved to still host a planet at such a 'short' orbital distance,
while from the exoplanet analysis we know that the planet is there!"

- By performing extensive numerical
simulations, the team thinks that star-planet tides might have brought
the planet inward from its original, wider orbit, placing it where we
see it today.

- Adibekyan adds: "The solution to
this scientific dilemma is hidden in the 'simple fact' that stars and
their planets not only form but also evolve together. In this
particular case, the planet managed to avoid engulfment."

- In the past decade,
asteroseismology has had a significant impact on the study of
solar-type and red-giant stars, which exhibit convection-driven,
solar-like oscillations. These studies have advanced considerably with
space-based observatories like CoRoT (CNES/ESA) and Kepler (NASA), and
are set to continue in the next decade with TESS and PLATO (ESA).

- Tiago Campante explains that:
"IA's involvement in TESS is at the level of the scientific
coordination within the TESS Asteroseismic Science Consortium (TASC).
TASC is a large and unique scientific collaboration, bringing together
all relevant research groups and individuals from around the world who
are actively engaged in research in the field of asteroseismology.
Following in the footsteps of its successful predecessor, the Kepler
Asteroseismic Science Consortium (KASC), TASC is based on a
collaborative and transparent working-group structure, aimed at
facilitating open collaboration between scientists."

• September 26, 2019: For the
first time, NASA’s planet-hunting TESS watched a black hole tear
apart a star in a cataclysmic phenomenon called a tidal disruption
event. Follow-up observations by NASA’s Neil Gehrels Swift Observatory
and other facilities have produced the most detailed look yet at the
early moments of one of these star-destroying occurrences. 27)

- “TESS data let us see
exactly when this destructive event, named ASASSN-19bt, started to get
brighter, which we’ve never been able to do before,” said
Thomas Holoien, a Carnegie Fellow at the Carnegie Observatories in
Pasadena, California. “Because we identified the tidal disruption
quickly with the ground-based All-Sky Automated Survey for Supernovae (ASAS-SN),
we were able to trigger multiwavelength follow-up observations in the
first few days. The early data will be incredibly helpful for modeling
the physics of these outbursts.”

Figure 20: This illustration
shows a tidal disruption, which occurs when a passing star gets too
close to a black hole and is torn apart into a stream of gas. Some of
the gas eventually settles into a structure around the black hole
called an accretion disk (image credit: NASA's Goddard Space Flight
Center)

Figure 21: When a star strays
too close to a black hole, intense tides break it apart into a stream
of gas. The tail of the stream escapes the system, while the rest of it
swings back around, surrounding the black hole with a disk of debris.
This video includes images of a tidal disruption event called
ASASSN-19bt taken by NASA’s TESS and Swift missions, as well as
an animation showing how the event unfolded (video credit: NASA's
Goddard Space Flight Center)

- A paper describing the findings,
led by Holoien, was published in the Sept. 27, 2019, issue of The
Astrophysical Journal and is now available online. 28)

- ASAS-SN, a worldwide network of
20 robotic telescopes headquartered at Ohio State University (OSU) in
Columbus, discovered the event on Jan. 29. Holoien was working at the Las Campanas Observatory
in Chile when he received the alert from the project’s South
Africa instrument. Holoien quickly trained two Las Campanas telescopes
on ASASSN-19bt and then requested follow-up observations by Swift, ESA’s (European Space Agency’s) XMM-Newton and ground-based 1-meter telescopes in the global Las Cumbres Observatory network.

- TESS, however,
didn’t need a call to action because it was already looking at
the same area. The planet hunter monitors large swaths of the sky,
called sectors, for 27 days at a time. This lengthy view allows TESS to
observe transits, periodic dips in a star’s brightness that may
indicate orbiting planets.

- ASAS-SN began spending more time
looking at TESS sectors when the satellite started science operations
in July 2018. Astronomers anticipated TESS could catch the earliest
light from short-lived stellar outbursts, including supernovae and
tidal disruptions. TESS first saw ASASSN-19bt on Jan. 21, over a week
before the event was bright enough for ASAS-SN to detect it. However,
the satellite only transmits data to Earth every two weeks, and once
received they must be processed at NASA’s Ames Research Center in
Silicon Valley, California. So the first TESS data on the tidal
disruption were not available until March 13. This is why obtaining
early follow-up observations of these events depends on coordination by
ground-based surveys like ASAS-SN.

- Fortunately, the disruption also
occurred in TESS’s southern continuous viewing zone, which was
always in sight of one of the satellite’s four cameras. (TESS
shifted to monitoring the northern sky at
the end of July.) ASASSN-19bt’s location allowed Holoien and his
colleagues to follow the event across several sectors. If it had
occurred outside this zone, TESS might have missed the beginning of the
outburst.

- “The early TESS data allow
us to see light very close to the black hole, much closer than
we’ve been able to see before,” said Patrick Vallely, a
co-author and National Science Foundation Graduate Research Fellow at
OSU. “They also show us that ASASSN-19bt’s rise in
brightness was very smooth, which helps us tell that the event was a
tidal disruption and not another type of outburst, like from the center
of a galaxy or a supernova.”

- Holoien’s team used UV data
from Swift — the earliest yet seen from a tidal disruption
— to determine that the temperature dropped by about 50%, from
around 71,500 to 35,500 degrees Fahrenheit (40,000 to 20,000 º
Celsius), over a few days. It’s the first time such an early
temperature decrease has been seen in a tidal disruption before,
although a few theories have predicted it, Holoien said.

- More typical for these kinds of
events was the low level of X-ray emission seen by both Swift and
XMM-Newton. Scientists don’t fully understand why tidal
disruptions produce so much UV emission and so few X-rays.

- “People have suggested
multiple theories — perhaps the light bounces through the newly
created debris and loses energy, or maybe the disk forms further from
the black hole than we originally thought and the light isn’t so
affected by the object’s extreme gravity,” said S. Bradley
Cenko, Swift’s principal investigator at NASA’s Goddard
Space Flight Center in Greenbelt, Maryland. “More early-time
observations of these events may help us answer some of these lingering
questions.”

- Astronomers think the
supermassive black hole that generated ASASSN-19bt weighs around 6
million times the Sun’s mass. It sits at the center of a galaxy
called 2MASX J07001137-6602251
located around 375 million light-years away in the constellation
Volans. The destroyed star may have been similar in size to our Sun.

- Tidal disruptions are incredibly
rare, occurring once every 10,000 to 100,000 years in a galaxy the size
of our own Milky Way. Supernovae, by comparison, happen every 100 years
or so. In total, astronomers have observed only about 40 tidal
disruptions so far, and scientists predicted TESS would see only one or
two in its initial two-year mission.

- “For TESS to observe
ASASSN-19bt so early in its tenure, and in the continuous viewing zone
where we could watch it for so long, is really quite
extraordinary,” said Padi Boyd, the TESS project scientist at
Goddard. “Future collaborations with observatories around the
world and in orbit will help us learn even more about the different
outbursts that light up the cosmos.”

- TESS is a NASA Astrophysics
Explorer mission led and operated by MIT in Cambridge, Massachusetts,
and managed by NASA's Goddard Space Flight Center. Additional partners
include Northrop Grumman, based in Falls Church, Virginia; NASA’s
Ames Research Center in California’s Silicon Valley; the
Harvard-Smithsonian Center for Astrophysics in Cambridge,
Massachusetts; MIT’s Lincoln Laboratory; and the Space Telescope
Science Institute in Baltimore. More than a dozen universities,
research institutes and observatories worldwide are participants in the
mission.

- NASA's Goddard Space Flight
Center manages the Swift mission in collaboration with Penn State in
University Park, the Los Alamos National Laboratory in New Mexico and
Northrop Grumman Innovation Systems in Dulles, Virginia. Other partners
include the University of Leicester and Mullard Space Science
Laboratory of the University College London in the United Kingdom,
Brera Observatory and ASI.

• July 29, 2019: NASA’s newest planet hunter, the Transiting Exoplanet Survey Satellite (TESS),
has discovered three new worlds — one slightly larger than Earth
and two of a type not found in our solar system — orbiting a
nearby star. The planets straddle an observed gap in the sizes of known
planets and promise to be among the most curious targets for future
studies. 29)

- TESS Object of Interest (TOI) 270 is a faint, cool star more commonly identified by its catalog name: UCAC4 191-004642.
The M-type dwarf star is about 40% smaller than the Sun in both size
and mass, and it has a surface temperature about one-third cooler than
the Sun’s. The planetary system lies about 73 light-years away in
the southern constellation of Pictor.

Figure 22: This infographic
illustrates key features of the TOI 270 system, located about 73
light-years away in the southern constellation Pictor. The three known
planets were discovered by NASA’s Transiting Exoplanet Survey
Satellite through periodic dips in starlight caused by each orbiting
world. Insets show information about the planets, including their
relative sizes, and how they compare to Earth. Temperatures given for
TOI 270’s planets are equilibrium temperatures, calculated
without the warming effects of any possible atmospheres (image credit:
NASA’s Goddard Space Flight Center/Scott Wiessinger)

- “This system is exactly
what TESS was designed to find — small, temperate planets that
pass, or transit, in front of an inactive host star, one lacking
excessive stellar activity, such as flares,” said lead researcher
Maximilian Günther, a Torres Postdoctoral Fellow at the
Massachusetts Institute of Technology’s (MIT) Kavli Institute for
Astrophysics and Space Research in Cambridge. “This star is quiet
and very close to us, and therefore much brighter than the host stars
of comparable systems. With extended follow-up observations,
we’ll soon be able to determine the make-up of these worlds,
establish if atmospheres are present and what gases they contain, and
more.”

- A paper describing the system was published in the journal Nature Astronomy and is now available online. 30)

- The innermost planet, TOI 270 b,
is likely a rocky world about 25% larger than Earth. It orbits the star
every 3.4 days at a distance about 13 times closer than Mercury orbits
the Sun. Based on statistical studies of known exoplanets of similar
size, the science team estimates TOI 270 b has a mass around 1.9 times
greater than Earth’s.

- Due to its proximity to the star,
planet b is an oven-hot world. Its equilibrium temperature — that
is, the temperature based only on energy it receives from the star,
which ignores additional warming effects from a possible atmosphere
— is around 490 º Fahrenheit (254ºC).

- The other two
planets, TOI 270 c and d, are, respectively, 2.4 and 2.1 times larger
than Earth and orbit the star every 5.7 and 11.4 days. Although only
about half its size, both may be similar to Neptune in our solar
system, with compositions dominated by gases rather than rock, and they
likely weigh around 7 and 5 times Earth’s mass, respectively.

Figure 23: Compare and contrast
worlds in the TOI 270 system with these illustrations of each planet.
Temperatures given for TOI 270 planets are equilibrium temperatures,
calculated without taking into account the warming effects of any
possible atmospheres (image credit: NASA’s Goddard Space Flight
Center)

- All of the planets are expected
to be tidally locked to the star, which means they only rotate once
every orbit and keep the same side facing the star at all times, just
as the Moon does in its orbit around Earth.

- Planet c and d might best be
described as mini-Neptunes, a type of planet not seen in our own solar
system. The researchers hope further exploration of TOI 270 may help
explain how two of these mini-Neptunes formed alongside a nearly
Earth-size world.

- “An interesting aspect of
this system is that its planets straddle a well-established gap in
known planetary sizes,” said co-author Fran Pozuelos, a
postdoctoral researcher at the University of Liège in Belgium.
“It is uncommon for planets to have sizes between 1.5 and two
times that of Earth for reasons likely related to the way planets form,
but this is still a highly controversial topic. TOI 270 is an excellent
laboratory for studying the margins of this gap and will help us better
understand how planetary systems form and evolve.”

- Günther’s team is
particularly interested in the outermost planet, TOI 270 d. The team
estimates the planet’s equilibrium temperature to be about
150º Fahrenheit (66º C). This makes it the most temperate
world in the system — and as such, a rarity among known
transiting planets.

- "TOI 270 is perfectly situated in
the sky for studying the atmospheres of its outer planets with NASA's
future James Webb Space Telescope," said co-author Adina Feinstein, a
doctoral student at the University of Chicago. "It will be observable
by Webb for over half a year, which could allow for really interesting
comparison studies between the atmospheres of TOI 270 c and d."

Figure 24: The TOI 270 system is
so compact that the orbits of Jupiter and its moons in our own solar
system offer the closest reasonable comparison, as illustrated here
(image credit: NASA/GSFC (Goddard Space Flight Center))

- The team hopes further research
may reveal additional planets beyond the three now known. If planet d
has a rocky core covered by a thick atmosphere, its surface would be
too warm for the presence of liquid water, considered a key requirement
for a potentially habitable world. But follow-up studies may discover
additional rocky planets at slightly greater distances from the star,
where cooler temperatures could allow liquid water to pool on their
surfaces.

- TESS is a NASA Astrophysics
Explorer mission led and operated by MIT in Cambridge, Massachusetts,
and managed by NASA's Goddard Space Flight Center. Additional partners
include Northrop Grumman, based in Falls Church, Virginia; NASA’s
Ames Research Center in California’s Silicon Valley; the
Harvard-Smithsonian Center for Astrophysics in Cambridge,
Massachusetts; MIT’s Lincoln Laboratory; and the Space Telescope
Science Institute in Baltimore. More than a dozen universities,
research institutes and observatories worldwide are participants in the
mission.

• July 25, 2019: NASA’s
TESS (Transiting Exoplanet Survey Satellite) has discovered 21 planets
outside our solar system and captured data on other interesting events
occurring in the southern sky during its first year of science. TESS
has now turned its attention to the Northern Hemisphere to complete the
most comprehensive planet-hunting expedition ever undertaken. 31)

- TESS began
hunting for exoplanets (or worlds orbiting distant stars) in the
southern sky in July of 2018, while also collecting data on supernovae,
black holes and other phenomena in its line of sight. Along with the
planets TESS has discovered, the mission has identified over 850
candidate exoplanets that are waiting for confirmation by ground-based
telescopes.

- “The pace and productivity
of TESS in its first year of operations has far exceeded our most
optimistic hopes for the mission,” said George Ricker,
TESS’s principal investigator at the Massachusetts Institute of
Technology in Cambridge. “In addition to finding a diverse set of
exoplanets, TESS has discovered a treasure trove of astrophysical
phenomena, including thousands of violently variable stellar
objects.”

- To search for exoplanets, TESS
uses four large cameras to watch a 24-by-96-degree section of the sky
for 27 days at a time. Some of these sections overlap, so some parts of
the sky are observed for almost a year. TESS is concentrating on stars
closer than 300 light-years from our solar system, watching for
transits, which are periodic dips in brightness caused by an object,
like a planet, passing in front of the star.

Figure 25: Here are highlights
from TESS's first year of science operations. All exoplanet animations
are illustrations. To search for exoplanets, TESS uses four large
cameras to watch a 24 x 96 degree section of the sky for 27 days at a
time. Some of these sections overlap, so some parts of the sky are
observed for almost a year. TESS is concentrating on stars closer than
300 light-years from our solar system, watching for transits, which are
periodic dips in brightness caused by an object, like a planet, passing
in front of the star (video credit: NASA/GSFC, Published on 25 July
2019)

- On July 18, the southern portion
of the survey was completed and the spacecraft turned its cameras to
the north. When it completes the northern section in 2020, TESS will
have mapped over three quarters of the sky.

- “Kepler discovered the
amazing result that, on average, every star system has a planet or
planets around it,” said Padi Boyd, TESS project scientist at
NASA’s Goddard Space Flight Center in Greenbelt, Maryland.
“TESS takes the next step. If planets are everywhere, let’s
find those orbiting bright, nearby stars because they’ll be the
ones we can now follow up with existing ground and space-based
telescopes, and the next generation of instruments for decades to
come.”

- Here are a few of the interesting objects and events TESS saw during its first year.

Exoplanets

- To qualify as an exoplanet
candidate, an object must make at least three transits in the TESS
data, and then pass through several additional checks to make sure the
transits were not a false positive caused by an eclipse or companion
star, but may in fact be an exoplanet. Once a candidate is identified,
astronomers deploy a large network of ground-based telescopes to
confirm it.

- “The team is currently
focused on finding the best candidates to confirm by ground-based
follow-up,” said Natalia Guerrero, who manages the team in charge
of identifying exoplanet candidates at MIT. “But there are many
more potential exoplanet candidates in the data yet to be analyzed, so
we’re really just seeing the tip of the iceberg here. TESS has
only scratched the surface.”

- The planets TESS has discovered
so far range from a world 80% the size of Earth to ones comparable to
or exceeding the sizes of Jupiter and Saturn. Like Kepler, TESS is
finding many planets smaller in size than Neptune, but larger than
Earth.

- While NASA is striving to put
astronauts on some of our nearest neighbors — the Moon and Mars
— in order to understand more about the planets in our own solar
system, follow-up observations with powerful telescopes of the planets
TESS discovers will enable us to better understand how Earth and the
solar system formed.

- With TESS’s data,
scientists using current and future observatories, like the James Webb
Space Telescope, will be able to study other aspects of exoplanets,
like the presence and composition of any atmosphere, which would impact
the possibility of developing life.

Comets

- Before science operations
started, TESS snapped clear images of a newly discovered comet in our
solar system. During on-orbit instrument testing, the satellite’s
cameras took a series of images that captured the motion of C/2018 N1,
a comet found on June 29 by NASA’s Near-Earth Object Wide-field
Infrared Survey Explorer (NEOWISE).

- TESS captured data on similar objects outside the solar system as well.

Exocomets

- Data from the mission were also
used to identify transits by comets orbiting another star: Beta
Pictoris, located 63 light-years away. Astronomers were able to find
three comets that were too small to be planets and had detectable
tails, the first identification of its type in visible light.

Supernovae

- Because TESS
spends nearly a month looking in the same location, it can capture data
on stellar events, like supernovae, as they begin. During its first
months of science operations, TESS spotted six supernovae occurring in distant galaxies that were later discovered by ground-based telescopes.

- Scientists hope to use these
types of observations to better understand the origins of a specific
kind of explosion known as a Type Ia supernova.

- Type Ia supernovae occur either
in star systems where one white dwarf draws gas from another star or
when two white dwarfs merge. Astronomers don’t know which case is
more common, but with data from TESS, they’ll have a clearer
understanding of the origins of these cosmic blasts.

- Type Ia supernovae are a class of
objects called a “standard candle,” meaning astronomers
know how luminous they are and can use them to calculate quantities
like how quickly the universe is expanding. TESS data will help them
understand differences between Type Ia supernovae created in both
circumstances, which could have a large impact on how we understand
events happening billions of light-years away and, ultimately, the fate
of the universe.

• June 27, 2019: NASA’s
Transiting Exoplanet Survey Satellite (TESS) has discovered a world
between the sizes of Mars and Earth orbiting a bright, cool, nearby
star. The planet, called L 98-59b, marks the tiniest planet discovered
by TESS to date. 32)

- Two other worlds orbit the same
star. While all three planets’ sizes are known, further study
with other telescopes will be needed to determine if they have
atmospheres and, if so, which gases are present. The L 98-59 worlds
nearly double the number of small exoplanets — that is, planets beyond our solar system — that have the best potential for this kind of follow-up.

Figure 26: The three planets
discovered in the L98-59 system by NASA’s TESS mission are
compared to Mars and Earth in order of increasing size in this
illustration (image credit: NASA/GSFC)

- “The discovery is a great
engineering and scientific accomplishment for TESS,” said Veselin
Kostov, an astrophysicist at NASA’s Goddard Space Flight Center
in Greenbelt, Maryland, and the SETI Institute in Mountain View,
California. “For atmospheric studies of small planets, you need
short orbits around bright stars, but such planets are difficult to
detect. This system has the potential for fascinating future
studies.” 33)

- L 98-59b is around 80%
Earth’s size and about 10% smaller than the previous record
holder discovered by TESS. Its host star, L 98-59, is an M dwarf about one-third the mass of the Sun and lies about 35 light-years away in the southern constellation Volans.
While L 98-59b is a record for TESS, even smaller planets have been
discovered in data collected by NASA’s Kepler satellite,
including Kepler-37b, which is only 20% larger than the Moon.

- The two other
worlds in the system, L 98-59c and L 98-59d, are respectively around
1.4 and 1.6 times Earth’s size. All three were discovered by TESS
using transits, periodic dips in the star’s brightness caused
when each planet passes in front of it.

- TESS monitors one 24-by-96-degree
region of the sky, called a sector, for 27 days at a time. When the
satellite finishes its first year of observations in July, the L 98-59
system will have appeared in seven of the 13 sectors that make up the
southern sky. Kostov’s team hopes this will allow scientists to
refine what’s known about the three confirmed planets and search
for additional worlds.

- “If you have more than one
planet orbiting in a system, they can gravitationally interact with
each other,” said Jonathan Brande, a co-author and astrophysicist
at Goddard and the University of Maryland, College Park. “TESS
will observe L 98-59 in enough sectors that it may be able to detect
planets with orbits around 100 days. But if we get really lucky, we
might see the gravitational effects of undiscovered planets on the ones
we currently know.”

- M dwarfs like L 98-59 account for
three-quarters of our Milky Way galaxy’s stellar population. But
they are no larger than about half the Sun’s mass and are much
cooler, with surface temperatures less than 70% of the Sun’s.
Other examples include TRAPPIST-1, which hosts a system of seven
Earth-size planets, and Proxima Centauri, our nearest stellar neighbor,
which has one confirmed planet. Because these small, cool stars are so
common, scientists want to learn more about the planetary systems that
form around them.

- L 98-59b, the innermost world,
orbits every 2.25 days, staying so close to the star it receives as
much as 22 times the amount of energy Earth receives from the Sun. The
middle planet, L 98-59c, orbits every 3.7 days and experiences about 11
times as much radiation as Earth. L 98-59d, the farthest planet
identified in the system so far, orbits every 7.5 days and is blasted
with around four times the radiant energy as Earth.

- None of the planets lie within the star’s “habitable zone,”
the range of distances from the star where liquid water could exist on
their surfaces. However, all of them occupy what scientists call the Venus zone,
a range of stellar distances where a planet with an initial Earth-like
atmosphere could experience a runaway greenhouse effect that transforms
it into a Venus-like atmosphere. Based on its size, the third planet
could be either a Venus-like rocky world or one more like Neptune, with a small, rocky core cocooned beneath a deep atmosphere.

- One of TESS’s goals is to
build a catalog of small, rocky planets on short orbits around very
bright, nearby stars for atmospheric study by NASA's upcoming James Webb Space Telescope. Four of the TRAPPIST-1 worlds are prime candidates, and Kostov’s team suggests the L 98-59 planets are as well.

- The TESS mission feeds our desire to understand where we came from and whether we’re alone in the universe.

- "If we viewed the Sun from L
98-59, transits by Earth and Venus would lead us to think the planets
are almost identical, but we know they’re not,” said Joshua
Schlieder, a co-author and an astrophysicist at Goddard. “We
still have many questions about why Earth became habitable and Venus
did not. If we can find and study similar examples around other stars,
like L 98-59, we can potentially unlock some of those secrets.”

• April 15, 2019: NASA’s
TESS (Transiting Exoplanet Survey Satellite) has discovered its first
Earth-size world. The planet, HD 21749c, is about 89% Earth’s
diameter. It orbits HD 21749, a K-type star with about 70% of the
Sun’s mass located 53 light-years away in the southern
constellation Reticulum, and is the second planet TESS has identified
in the system. The new world is likely rocky and circles very close to
its star, completing one orbit in just under eight days. The planet is
likely very hot, with surface temperatures perhaps as high as 800º
F (427 ºC). 34)

- This is the 10th confirmed planet discovered by TESS, and hundreds of additional candidates are now being studied.

- Scientists at MIT (Massachusetts
Institute of Technology) and the Carnegie Institution for Science
analyzed TESS transit data from the first four sectors of TESS
observations to detect 11 periodic dips in the star’s brightness.
From this, they determined that the star’s light was being
partially blocked by a planet about the size of Earth.

- The star that HD 21749c orbits is
bright and relatively nearby, and therefore well suited to more
detailed follow-up studies, which could provide critical information
about the planet’s properties, including potentially the first
mass measurement of an Earth-size planet found by TESS. 35)

• March 26, 2019: NASA's new
TESS (Transiting Exoplanet Survey Satellite) is designed to ferret out
habitable exoplanets, but with hundreds of thousands of sunlike and
smaller stars in its camera views, which of those stars could host
planets like our own? A team of astronomers has identified the most
promising targets for this search. 36)37)

- TESS will observe 400,000 stars
across the whole sky to catch a glimpse of a planet transiting across
the face of its star, one of the primary methods by which exoplanets
are identified.

- A team of
astronomers from Cornell University, Lehigh University and Vanderbilt
University has identified the most promising targets for this search in
the new "TESS Habitable Zone Star Catalog," published in Astrophysical
Journal Letters. Lead author is Lisa Kaltenegger, professor of
astronomy at Cornell, director of Cornell's Carl Sagan Institute and a
member of the TESS science team. 38)

- The catalog identifies 1,822
stars for which TESS is sensitive enough to spot Earth-like planets
just a bit larger than Earth that receive radiation from their star
equivalent to what Earth receives from our sun. For 408 stars, TESS can
glimpse a planet just as small as Earth, with similar irradiation, in
one transit alone.

- "Life could exist on all sorts of
worlds, but the kind we know can support life is our own, so it makes
sense to first look for Earth-like planets," Kaltenegger said. "This
catalog is important for TESS because anyone working with the data
wants to know around which stars we can find the closest
Earth-analogs."

- Kaltenegger leads a program on
TESS that is observing the catalog's 1,822 stars in detail, looking for
planets. "I have 408 new favorite stars," said Kaltenegger. "It is
amazing that I don't have to pick just one; I now get to search
hundreds of stars."

- Confirming an exoplanet has been
observed and figuring out the distance between it and its star requires
detecting two transits across the star. The 1,822 stars the researchers
have identified in the catalog are ones from which TESS could detect
two planetary transits during its mission. Those orbital periods place
them squarely in the habitable zone of their star.

- The habitable zone is the area
around a star at which water can be liquid on a rocky planet's surface,
therefore considered ideal for sustaining life. As the researchers
note, planets outside the habitable zone could certainly harbor life,
but it would be extremely difficult to detect any signs of life on such
frozen planets without flying there.

- The catalog also identifies a
subset of 227 stars for which TESS can not only probe for planets that
receive the same irradiation as Earth, but for which TESS can also
probe out farther, covering the full extent of the habitable zone all
the way to cooler Mars-like orbits. This will allow astronomers to
probe the diversity of potentially habitable worlds around hundreds of
cool stars during the TESS mission's lifetime.

- The stars selected for the
catalog are bright, cool dwarfs, with temperatures roughly between
2,700 and 5,000 degrees Kelvin. The stars in the catalog are selected
due to their brightness; the closest are only approximately 6
light-years from Earth.

- "We don't know how many planets
TESS will find around the hundreds of stars in our catalog or whether
they will be habitable," Kaltenegger said, "but the odds are in our
favor. Some studies indicate that there are many rocky planets in the
habitable zone of cool stars, like the ones in our catalog. We're
excited to see what worlds we'll find."

- A total of 137 stars in the
catalog are within the continuous viewing zone of NASA's James Webb
Space Telescope, now under construction. Webb will be able to observe
them to characterize planetary atmospheres and search for signs of life
in their atmospheres.

- Planets TESS identifies may also
make excellent targets for observations by ground-based extremely large
telescopes currently being built, the researchers note, as the
brightness of their host stars would make them easier to characterize.

- "This is a remarkable time in
human history and a huge leap for our understanding of our place in the
universe," said Stassun, a member of the TESS science team.

• January 7, 2019: NASA's TESS
mission has discovered a third small planet outside our solar system,
scientists announced this week at the annual AAS (American Astronomical
Society) meeting in Seattle, WA. 39)40)41)

Figure 27: NASA’s TESS
mission, which will survey the entire sky over the next two years, has
already discovered three new exoplanets around nearby stars (image
credit: NASA/GSFC, edited by MIT News)

- The new planet, named HD 21749b,
orbits a bright, nearby dwarf star about 53 light years away, in the
constellation Reticulum, and appears to have the longest orbital period
of the three planets so far identified by TESS. HD 21749b journeys
around its star in a relatively leisurely 36 days, compared to the two
other planets — Pi Mensae b, a “super-Earth” with a
6.3-day orbit, and LHS 3844b, a rocky world that speeds around its star
in just 11 hours. All three planets were discovered in the first three
months of TESS observations.

- The surface of the new planet is
likely around 300 degrees Fahrenheit (150ºC) — relatively
cool, given its proximity to its star, which is almost as bright as the
sun.

- “It’s the coolest
small planet that we know of around a star this bright,” says
Diana Dragomir, a postdoc in MIT’s Kavli Institute for
Astrophysics and Space Research, who led the new discovery. “We
know a lot about atmospheres of hot planets, but because it’s
very hard to find small planets that orbit farther from their stars,
and are therefore cooler, we haven’t been able to learn much
about these smaller, cooler planets. But here we were lucky, and caught
this one, and can now study it in more detail.”

- The planet is
about three times the size of Earth, which puts it in the category of a
“sub-Neptune.” Surprisingly, it is also a whopping 23 times
as massive as the Earth. But it is unlikely that the planet is rocky
and therefore habitable; it’s more likely made of gas, of a kind
that is much more dense than the atmospheres of either Neptune or
Uranus.

- “We think this planet
wouldn’t be as gaseous as Neptune or Uranus, which are mostly
hydrogen and really puffy,” Dragomir says. “The planet
likely has a density of water, or a thick atmosphere.”

- Serendipitously, the researchers
have also detected evidence of a second planet, though not yet
confirmed, in the same planetary system, with a shorter, 7.8-day orbit.
If it is confirmed as a planet, it could be the first Earth-sized
planet discovered by TESS.

- Since TESS launched in April
2018, the spacecraft has been monitoring the sky, sector by sector, for
momentary dips in the light of about 200,000 nearby stars. Such dips
likely represent a planet passing in front of that star.

- The satellite trains its four
onboard cameras on each sector for 27 days, taking in light from the
stars in that particular segment before shifting to view the next one.
Over its two-year mission, TESS will survey nearly the entire sky by
monitoring and piecing together overlapping slices of the night sky.
The satellite will spend the first year surveying the sky in the
Southern Hemisphere, before swiveling around to take in the Northern
Hemisphere sky.

- The mission has released to the
public all the data TESS has collected so far from the first three of
the 13 sectors that it will monitor in the southern sky. For their new
analysis, the researchers looked through this data, collected between
July 25 and Oct. 14.

- Within the sector 1 data,
Dragomir identified a single transit, or dip, in the light from the
star HD 21749. As the satellite only collects data from a sector for 27
days, it’s difficult to identify planets with orbits longer than
that time period; by the time a planet passes around again, the
satellite may have shifted to view another slice of the sky.

- To complicate matters, the star
itself is relatively active, and Dragomir wasn’t sure if the
single transit she spotted was a result of a passing planet or a blip
in stellar activity. So she consulted a second dataset, collected by
the HARPS (High Accuracy Radial velocity Planet Searcher), a
high-precision spectrograph installed on a large ground-based telescope
in Chile, which identifies exoplanets by their gravitational tug on
their host stars.

- “They had looked at this
star system a decade ago and never announced anything because they
weren’t sure if they were looking at a planet versus the activity
of the star,” Dragomir says. “But we had this one transit,
and knew something was there.”

Stellar detectives

- When the researchers looked
through the HARPS data, they discovered a repeating signal emanating
from HD 21749 every 36 days. From this, they estimated that, if they
indeed had seen a transit in the TESS data from sector 1, then another
transit should appear 36 days later, in data from sector 3. When that
data became publicly available, a momentary glitch created a gap in the
data just at the time when Dragomir expected the second transit to
occur.

- “Because there was an
interruption in data around that time, we initially didn’t see a
second transit, and were pretty disappointed,” Dragomir recalls.
“But we re-extracted the data and zoomed in to look more
carefully, and found what looked like the end of a transit.”

- She and her colleagues compared
the pattern to the first full transit they had originally discovered,
and found a near perfect match — an indication that the planet
passed again in front of its star, in a 36-day orbit.

- “There was quite some
detective work involved, and the right people were there at the right
time,” Dragomir says. “But we were lucky and we caught the
signals, and they were really clear.”

- They also used data from the
Planet Finder Spectrograph, an instrument installed on the Magellan
Telescope in Chile, to further validate their findings and constrain
the planet’s mass and orbit.

- Once TESS has completed its
two-year monitoring of the entire sky, the science team has committed
to delivering information on 50 small planets less than four times the
size of Earth to the astronomy community for further follow-up, either
with ground-based telescopes or the future James Webb Space Telescope.

- “We’ve confirmed
three planets so far, and there are so many more that are just waiting
for telescope and people time to be confirmed,” Dragomir says.
“So it’s going really well, and TESS is already helping us
to learn about the diversity of these small planets.”

- TESS is a NASA Astrophysics Explorer mission led and operated by MIT
in Cambridge, Massachusetts, and managed by Goddard. Additional
partners include Northrop Grumman, based in Falls Church, Virginia;
NASA’s Ames Research Center in California’s Silicon Valley;
the Harvard-Smithsonian Center for Astrophysics in Cambridge,
Massachusetts; MIT Lincoln Laboratory; and the Space Telescope Science
Institute in Baltimore. More than a dozen universities, research
institutes, and observatories worldwide are participants in the
mission.

• On 17 September 2018, TESS
(Transiting Exoplanet Survey Satellite) shared its first science
observations. Part of the data from TESS’s initial science orbit
includes a detailed picture of the southern sky
taken with all four of the planet-hunter’s wide-field cameras.
The image captures a wealth of stars and other objects, including
systems previously known to have exoplanets, planets beyond our solar
system. TESS will spend the next two years monitoring the nearest,
brightest stars for periodic dips in their brightness, known as
transits. Such transits suggest a planet may be passing in front of its
parent star. TESS is expected to find thousands of new planets using
this method. 42)43)

- TESS’s
cameras, designed and built by MIT’s Lincoln Laboratory in
Lexington, Massachusetts, and the MIT Kavli Institute, monitor large
swaths of the sky to look for transits. Transits occur when a planet
passes in front of its star as viewed from the satellite’s
perspective, causing a regular dip in the star’s brightness.

- TESS will spend two years
monitoring 26 such sectors for 27 days each, covering 85 percent of the
sky. During its first year of operations, the satellite will study the
13 sectors making up the southern sky. Then TESS will turn to the 13
sectors of the northern sky to carry out a second year-long survey.

Figure 30: TESS took this
snapshot of the Large Magellanic Cloud (right) and the bright star R
Doradus (left) with just a single detector of one of its cameras on 7
Aug. 2018. The frame is part of a swath of the southern sky TESS
captured in its “first light” science image as part of its
initial round of data collection (image credit: NASA/MIT/TESS)

Figure 31: TESS captured this
strip of stars and galaxies in the southern sky during one 30-minute
period on 7 Aug. 2018. Created by combining the view from all four of
its cameras, this is TESS’s “first light,” from the
first observing sector that will be used for identifying planets around
other stars. Notable features in this swath of the southern sky include
the Large and Small Magellanic Clouds and a globular cluster called NGC
104, also known as 47 Tucanae. The brightest stars in the image, Beta
Gruis and R Doradus, saturated an entire column of camera detector
pixels on the satellite’s second and fourth cameras (image
credit: NASA/MIT/TESS)

Figure 32: How NASA’s
newest planet hunter scans the sky. This animation shows how TESS will
study 85 percent of the sky in 26 sectors. The spacecraft will observe
the 13 sectors that make up the southern sky in the first year and the
13 sectors of the northern sky in the second year (video credit:
NASA/GSFC)

• December 6, 2018: The first batch of TESS mission data is now available through MAST.
This release includes all data from Sectors 1 and 2, observed between
July 25 and September 20, 2018. This includes both FFI (Full Frame
Images) and 2-min cadence data. 44)

Figure 33: Map of observations (image credit: STScI)

• August 6, 2018: Before
NASA’s TESS started science operations on July 25, 2018, the
planet hunter sent back a stunning sequence of serendipitous images
showing the motion of a comet. Taken over the course of 17 hours on
July 25, these TESS images helped demonstrate the satellite’s
ability to collect a prolonged set of stable periodic images covering a
broad region of the sky — all critical factors in finding
transiting planets orbiting nearby stars. 45)

- Over the course
of these tests, TESS took images of C/2018 N1, a comet discovered by
NASA’s NEOWISE (Near-Earth Object Wide-field Infrared Survey
Explorer) satellite on June 29. The comet, located about 29 million
miles (48 million km) from Earth in the southern constellation Piscis
Austrinus, is seen to move across the frame from right to left as it
orbits the Sun. The comet’s tail, which consists of gases carried
away from the comet by an outflow from the Sun called the solar wind,
extends to the top of the frame and gradually pivots as the comet
glides across the field of view.

Figure 34: The animated gif
sequence is compiled from a series of images taken on July 25 by TESS.
The angular extent of the widest field of view is six degrees. Visible
in the images are the comet C/2018 N1, asteroids, variable stars,
asteroids and reflected light from Mars. TESS is expected to find
thousands of planets around other nearby stars (image credit: MIT,
NASA/GSFC)

- In addition to the comet, the
images reveal a treasure trove of other astronomical activity. The
stars appear to shift between white and black as a result of image
processing. The shift also highlights variable stars — which
change brightness either as a result of pulsation, rapid rotation, or
by eclipsing binary neighbors. Asteroids in our solar system appear as
small white dots moving across the field of view. Towards the end of
the video, one can see a faint broad arc of light moving across the
middle section of the frame from left to right. This is stray light
from Mars, which is located outside the frame. The images were taken
when Mars was at its brightest near opposition, or its closest
distance, to Earth.

- These images were taken during a
short period near the end of the mission’s commissioning phase,
prior to the start of science operations. The movie presents just a
small fraction of TESS’s active field of view. The team continues
to fine-tune the spacecraft’s performance as it searches for
distant worlds.

• July 27, 2018: NASA’s TESS
(Transiting Exoplanet Survey Satellite) has started its search for
planets around nearby stars, officially beginning science operations on
July 25, 2018. TESS is expected to transmit its first series of
science data back to Earth in August, and thereafter periodically every
13.5 days, once per orbit, as the spacecraft makes it closest approach
to Earth. The TESS Science Team will begin searching the data for new
planets immediately after the first series arrives. 46)

- “I’m thrilled that
our new planet hunter mission is ready to start scouring our solar
system’s neighborhood for new worlds,” said Paul Hertz,
NASA Astrophysics division director at Headquarters, Washington.
“Now that we know there are more planets than stars in our
universe, I look forward to the strange, fantastic worlds we’re
bound to discover.”

- TESS is NASA’s latest
satellite to search for planets outside our solar system, known as
exoplanets. The mission will spend the next two years monitoring the
nearest and brightest stars for periodic dips in their light. These
events, called transits, suggest that a planet may be passing in front
of its star. TESS is expected to find thousands of planets using this
method, some of which could potentially support life.

• July 11, 2018: After a
successful launch on April 18, 2018, NASA’s newest planet hunter,
the Transiting Exoplanet Survey Satellite, is currently undergoing a
series of commissioning tests before it begins searching for planets.
The TESS team has reported that the spacecraft and cameras are in good
health, and the spacecraft has successfully reached its final science
orbit. The team continues to conduct tests in order to optimize
spacecraft performance with a goal of beginning science at the end of
July. 47)

- Every new mission goes through a
commissioning period of testing and adjustments before beginning
science operations. This serves to test how the spacecraft and its
instruments are performing and determines whether any changes need to
be made before the mission starts observations.

• May 21, 2018: TESS
successfully completed a lunar flyby on 17 May at 06:34:35 UTC (2:34 AM
EST). At its closest approach, TESS was 8,253 km from the lunar
surface. Based on the successful lunar fly-by, no adjustment burn was
required. 48)

Figure 35: An artist’s
illustration of TESS as it passed the Moon during its lunar flyby. This
provided a gravitational boost that placed TESS on course for its final
working orbit (image credit: NASA's Goddard Space Flight Center)

- As part of commissioning, the
TESS science team took a 2-second test exposure using one of four TESS
cameras, providing an exciting glimpse of the type of image expected
from each of TESS’ four cameras. The image is centered on the
southern constellation Centaurus with the bright star Beta Centauri is
visible at the lower left edge. 49)

- TESS will undergo one final
thruster burn on May 30 to enter its science orbit around Earth. This
highly elliptical orbit will maximize the amount of sky the spacecraft
can image, allowing it to continuously monitor large swaths of the sky.
TESS is expected to begin science operations in mid-June after reaching
this orbit and completing camera calibrations.

Figure 36: This test image from
one of the four cameras aboard TESS captures a swath of the southern
sky along the plane of our galaxy. More than 200,000 stars are visible
in this image. TESS is expected to cover more than 400 times the amount
of sky shown in this image when using all four of its cameras during
science operations. The image, which is centered in the constellation
Centaurus, includes dark tendrils from the Coal Sack Nebula and the
bright emission nebula Ced 122 (upper right).The bright star at bottom
center is Beta Centauri (image credit: NASA/MIT/TESS)

Sensor complement: (Four WFOV cameras)

The TESS payload consists of four
identical cameras and a DHU (Data Handling Unit). Each camera consists
of a lens assembly with seven optical elements, and a detector assembly
with four CCDs (Charge Coupled Devices) and their associated
electronics. All four cameras are mounted onto a single plate (Figure 37)
that is attached to the spacecraft, such that their FOVs are lined up
to form a rectangle measuring 24º x 96º on the sky. Four
elliptical holes in the plate allow shimless alignment of the four
cameras at the desired angles. 50)51)52)

Each of the four cameras features:

- WFOV (Wide Field of View) of 24º x 24º

- 100 mm effective pupil diameter

- Lens assembly with 7 optical elements

- Athermal design

- 600nm - 1000 nm bandpass

- 16.8 Mpixel, low-noise, low-power, MIT/LL CCID-80 detector.

Figure 37: Illustration of the four cameras mounted on a single plate (image credit: NASA, MIT)

Detector assembly: The focal
plane consists of four back-illuminated MIT/LL CCID-80 devices. The
CCID-80, developed for TESS, is a deep-depletion, frame-transfer CCD
with a full frame store. The device has four outputs; each output is
associated with an array of 512 (H) x 2048 (V) imaging pixels, for a
total imaging area of 2048 (H) x 2048 (V). The die size is 32 (W) x 64
(H) mm for an area of 20.4 cm2. 53)

The imaging array, frame store, and
serial registers all consist of conventional three-phase, 15 x 15
µm pixels. There is a three-phase charge injection register at
the top of the array, and the serial register support bidirectional
transfer. The pixel array employs a trough design feature to provide
radiation mitigation for small charge packets. To enable the desired
fast frame transfer time, the image array and frame store clocks are
strapped with metal interconnect to reduce the RC delay from the clock
lines. The output circuit is a single-stage MOSFET similar to others
demonstrated at Lincoln Laboratory.

Figure 38: The detector assembly
of one of the prototype lenses. The light shield cover for the frame
store regions is removed (image credit: MIT/LL)

Lincoln Laboratory supports several
different styles of back-illumination processing. For TESS, a flow is
used that involves: epoxy mounting the device wafer to a support wafer;
wet chemical thinning the high resistivity float zone silicon to the
100 µm full depletion target; back-side passivation through an
ion implantation, laser annealing sequence; deposition and patterning
of antireflection and light shield coatings; and etches to provide
access to the bond pads.

The true benefit of the 100 µm thick detector is shown in the Figure 39
spectral response curve. The project observed over 20% improvement in
quantum efficiency at 1000 nm measurement wavelength over a 45 µm
thick device. - A total of sixteen CCDs arranged in four mosaics will
be needed.

Each of the four identical TESS
lenses is an f=1:4 custom design consisting of seven optical elements,
with an entrance pupil diameter of 10.5 cm (Figures 40 and 41).
For ease of manufacture, all lens surfaces are spherical except for two
mild aspheres. There are two separate aluminum lens barrels that are
fastened and pinned together. All optical elements have antireflection
coatings. The surface of one element also has a long-pass filter
coating to enforce the cutoff at 600 nm. The red limit at 1000 nm is
set by the quantum-efficiency curve of the CCDs (Figure 39).

Each lens forms a 24º x
24º unvignetted image on the four-CCD mosaic in its focal plane.
The optical design was optimized to provide small image spots of a
consistent size across the FOV (Field of View), and produce
undersampled images similar to those of Kepler. At nominal focus and
flight temperature (-75ºC), the 50% ensquared-energy half-width is
15 µm (one pixel or 0.35 arcmin) averaged over the FOV. Each lens
is equipped with a lens hood, which reduces the effects of scattered
light from the Earth and Moon (Ref. 3).

Figure 41:
Left: Two lens prototypes that were constructed. Right: The detector
assembly of one of the prototype lenses. The frame-store regions of the
CCDs are covered (image credit: NASA, TESS Team)

Scanning strategy: The four cameras act as a 1 x 4 array, providing a combined FOV of 24º x 96º or 2300 square degrees (Figure 42).
The north and south ecliptic hemispheres are each divided into 13
partially overlapping sectors of 24º x 96º, extending from an
ecliptic latitude of 6º to the ecliptic pole. Each sector is
observed continuously for two spacecraft orbits (27.4 days), with the
boresight of the four-camera array pointed nearly anti-solar. After two
orbits, the FOV is shifted eastward in ecliptic longitude by about
27º, to observe the next sector. Observing an entire hemisphere
takes one year, and the all-sky survey takes two years.

The overlap of the sectors is illustrated in Figure 42.
Approximately 30,000 square degrees are observed for at least 27 days.
Close to the ecliptic poles, approximately 2800 square degrees are
observed for more than 80 days. Surrounding the ecliptic poles,
approximately 900 square degrees are observed for more than 300 days.

Figure 42:
Left: The instantaneous combined FOV of the four TESS cameras. Middle:
Division of the celestial sphere into 26 observation sectors (13 per
hemisphere). Right: Duration of observations on the celestial sphere,
taking into account the overlap between sectors. The dashed black
circle enclosing the ecliptic pole shows the region which JWST will be
able to observe at any time (image credit: TESS Team)

Photometric performance: Figure 43
shows the anticipated photometric performance of the TESS cameras. The
noise sources in this model are photon-counting noise from the star and
the background (zodiacal light and faint unresolved stars), dark
current (negligible), readout noise, and a term representing additional
systematic errors that cannot be corrected by co-trending. The most
important systematic error is expected to be due to random pointing
variations (”spacecraft jitter"). Because of the non-uniform
quantum efficiency of the CCD pixels, motion of the star image on the
CCD will introduce changes in the measured brightness, as the weighting
of the image PSF (Point Spread Function) changes, and as parts of the
image PSF enter and exit the summed array of pixels.

The central pixel of a stellar image will saturate at approximately IC
= 7:5. However, this does not represent the bright limit for precise
photometry because the excess charge is spread across other CCD pixels
and is conserved, until the excess charge reaches the boundary of the
CCD. As long as the photometric aperture is large enough to encompass
all of the charge, high photometric precision can still be obtained.
The Kepler mission demonstrated that photon-noise{limited photometry
can be obtained for stars 4 mag brighter than the single-pixel
saturation limit. Since similar performance is expected for TESS, the
bright limit is expected to be IC~4 or perhaps even brighter.

Figure 43: Top: Expected 1σ photometric precision as a function of stellar apparent magnitude in the IC
band. Contributions are from photon-counting noise from the target star
and background (zodiacal light and unresolved stars), detector read
noise (10 e-), and an assumed 60 ppm of incorrigible noise
on hourly timescales. Bottom: The number of pixels in the photometric
aperture that optimizes the signal-to-noise ratio (image credit: TESS
Team)

The information compiled and edited in this article was provided byHerbert
J. Kramer from his documentation of: ”Observation of the Earth
and Its Environment: Survey of Missions and Sensors” (Springer
Verlag) as well as many other sources after the publication of the 4th
edition in 2002. - Comments and corrections to this article are always
welcome for further updates (herb.kramer@gmx.net).